[llvm] [PGO] Fix incorrect count threshold calculation when 0% cutoff (PR #117359)

[Ken Matsui via llvm-commits]

https://github.com/ken-matsui updated https://github.com/llvm/llvm-project/pull/117359

>From 5a2d689fcf182f03371ab6c48444c9282f3b14a6 Mon Sep 17 00:00:00 2001
From: Ken Matsui <ken.matsui at mediatek.com>
Date: Fri, 22 Nov 2024 10:34:49 -0500
Subject: [PATCH] [PGO] Fix incorrect count threshold calculation when 0%
 cutoff

DefaultCutoffsData does not have an entry for the 0th percentile.  As a
result, when the getEntryForPercentile method is called with a
percentile argument of 0, it returns a ProfileSummaryEntry for the 1st
percentile instead.  This behavior affects the threshold calculations,
such as getHotCountThreshold, causing them to incorrectly identify some
sample profile counts as hot when they should not be.

This patch addresses the issue by handling the 0th percentile case in
the getEntryForPercentile method.  This ensures that when the
-profile-summary-cutoff-hot (or -cold) option is set to 0, no sample
counts are treated as hot (or all sample counts are treated as cold).
---
 llvm/lib/ProfileData/ProfileSummaryBuilder.cpp | 9 +++++++++
 llvm/test/Analysis/ProfileSummary/basic.ll     | 8 ++++++++
 2 files changed, 17 insertions(+)

diff --git a/llvm/lib/ProfileData/ProfileSummaryBuilder.cpp b/llvm/lib/ProfileData/ProfileSummaryBuilder.cpp
index 3a45113b0a2eae..23c87702eb1335 100644
--- a/llvm/lib/ProfileData/ProfileSummaryBuilder.cpp
+++ b/llvm/lib/ProfileData/ProfileSummaryBuilder.cpp
@@ -77,9 +77,18 @@ static const uint32_t DefaultCutoffsData[] = {
 const ArrayRef<uint32_t> ProfileSummaryBuilder::DefaultCutoffs =
     DefaultCutoffsData;
 
+// An entry for the 0th percentile to correctly calculate hot/cold count
+// thresholds when -profile-summary-cutoff-hot/cold is 0.  If the hot cutoff is
+// 0, no sample counts are treated as hot.  If the cold cutoff is 0, all sample
+// counts are treated as cold.  Assumes there is no UINT64_MAX sample counts.
+static const ProfileSummaryEntry ZeroCutoffEntry = {0, UINT64_MAX, 0};
+
 const ProfileSummaryEntry &
 ProfileSummaryBuilder::getEntryForPercentile(const SummaryEntryVector &DS,
                                              uint64_t Percentile) {
+  if (Percentile == 0)
+    return ZeroCutoffEntry;
+
   auto It = partition_point(DS, [=](const ProfileSummaryEntry &Entry) {
     return Entry.Cutoff < Percentile;
   });
diff --git a/llvm/test/Analysis/ProfileSummary/basic.ll b/llvm/test/Analysis/ProfileSummary/basic.ll
index 966a1117c47d14..c4f48ccafde86d 100644
--- a/llvm/test/Analysis/ProfileSummary/basic.ll
+++ b/llvm/test/Analysis/ProfileSummary/basic.ll
@@ -2,12 +2,16 @@
 ; RUN: opt < %s -disable-output -profile-summary-hot-count=500 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=OVERRIDE-HOT
 ; RUN: opt < %s -disable-output -profile-summary-cold-count=0 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=OVERRIDE-COLD
 ; RUN: opt < %s -disable-output -profile-summary-cold-count=200 -profile-summary-hot-count=1000 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=OVERRIDE-BOTH
+; RUN: opt < %s -disable-output -profile-summary-cutoff-hot=0 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=HOT-CUTOFF-0
+; RUN: opt < %s -disable-output -profile-summary-cutoff-cold=0 -profile-summary-hot-count=18446744073709551615 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=COLD-CUTOFF-0
 
 define void @f1() !prof !20 {
 ; CHECK-LABEL: f1 :hot
 ; OVERRIDE-HOT-LABEL: f1
 ; OVERRIDE-COLD-LABEL: f1 :hot
 ; OVERRIDE-BOTH-LABEL: f1
+; HOT-CUTOFF-0-LABEL: f1{{$}}
+; COLD-CUTOFF-0-LABEL: f1 :cold
 
   ret void
 }
@@ -17,6 +21,8 @@ define void @f2() !prof !21 {
 ; OVERRIDE-HOT-LABEL: f2 :cold
 ; OVERRIDE-COLD-LABEL: f2
 ; OVERRIDE-BOTH-LABEL: f2
+; HOT-CUTOFF-0-LABEL: f2 :cold
+; COLD-CUTOFF-0-LABEL: f2 :cold
 
   ret void
 }
@@ -26,6 +32,8 @@ define void @f3() !prof !22 {
 ; OVERRIDE-HOT-LABEL: f3
 ; OVERRIDE-COLD-LABEL: f3
 ; OVERRIDE-BOTH-LABEL: f3
+; HOT-CUTOFF-0-LABEL: f3{{$}}
+; COLD-CUTOFF-0-LABEL: f3 :cold
 
   ret void
 }